The use of high molecular weight polyesters in thermoplastic molding compositions is well known to those skilled in the art. Such polyesters provide compositions with excellent moldability and molded articles therefrom having smooth and glossy surface appearance, high strength, stiffness, temperature resistance and other desirable properties.
However, a major short-coming in the use of such polyesters in molding compositions is their normally highly flammable nature. The flammability of polyesters has been reduced by using halogen-, phosphorus- or nitrogen-containing additives, and such compositions are described in various patents and publications. Flame retarded thermoplastic polyester compositions are desired for many fields of use such as in home construction, automobile and aircraft manufacture, packaging, electrical equipment and the like.
With particular reference to polyesters of the poly(alkylene dicarboxylate) family and related copolyesters, it has been difficult to adequately render such materials flame retardant without sacrificing their inherent superior toughness properties. Specifically, the use of conventional flame retardant additives in conventional amounts causes decreases in toughness, e.g., flex resistance and heat distortion resistance, especially in glass filled embodiments.
A highly useful family of flame retardant agents has been reported to comprise polybrominated diphenyl ether compounds, and particularly decabromodiphenyl ether. Patent describing such flame retardants are U.S. Pat. Nos. 3,624,024; 3,751,396; 3,873,491; 3,971,752; 4,010,219; and 4,070,332, the disclosures of which are incorporated herein by reference. Although diphenyl ethers having from 2 to 10 bromines are disclosed to be useful, e.g., in U.S. Pat. No. 4,010,219, the decabromodiphenyl ether analog has become the flame-retardant agent of commercial choice in poly(1,4-butylene terephthalate) resin compositions. Unfortunately, this compound has the undesirable habit of migrating to the surface of articles molded from compositions containing it. This habit, known as "blooming" or "plate-out", not only is visually unacceptable, but also causes breakdown in surface electrical characteristics of parts molded from compositions in which decabromodiphenyl ether is the flame-retardant agent.
It has now been discovered that if a small effective amount of an olefin polymer and/or olefin copolymer, e.g., polyethylene, is added to flame retardant polyester compositions wherein decabromodiphehyl ether is employed, compositions with a surprisingly reduced tendency to bloom when subjected to high temperatures over a period of time, are obtained. Furthermore, such compositions are provided without sacrificing the other desirable traits, such as toughness, of the polyester resins molding compositions of the prior art.
By way of illustration, the addition of a polyolefin resin additive to decabromodiphenyl ether/polyester blends provides compositions which are flame retardant to the point where they meet stringent Underwriter's Laboratory requirements for self-extinguishing compositions, but which at the same time hardly "bloom" or "plate out" at all on the surface of molded parts when subjected to oven aging at 100.degree., 140.degree., and 170.degree. C. for 1 hours, 24 hours and 168 hours. In contrast, under the same conditions, oven aging decabromodiphenyl ether polyester blends without polyolefin causes formation of a powder bloom on the surface, detrimentally affecting electrical properties.
It is, accordingly, a principal object of this invention to provide a family of bloom-resistant, flame retardant polyester compositions.
It is another object of this invention to provide flame retardant polyester compositions with excellent moldability and which, after oven aging, retain smooth and glossy surfaces, high flexural strength and, especially, superior surface electrical properties.
Other objects and advantages of the invention will become apparent from the following detailed description.